A fire or explosion can result from ignition of a combustible material, such as dust, gas, or vapor, when mixed with oxygen present in the environment. When such ignition takes place within a process or storage enclosure, or other system, the rapid rise in pressure developed may exert destructive forces within a few milliseconds, which may place both personnel and equipment at risk.
A number of industries may face the danger of ignition in an enclosed system, including plastics, food and dairy, pigments and dyes, wood processing, grain processing, coal processing, pharmaceuticals, grain ethanol, chemicals, metals, and agrochemicals. Within and/or beyond those industries, particular applications may pose the danger of such ignition. For example, cyclones, bag houses, cartridge filters, pneumatic conveying systems, milling processes (including pin milling, ball milling, etc.), bucket elevators, dryers, ovens, roller mills, grinding applications, and buildings may all pose the danger of ignition causing fire or explosion.
The destructive forces associated with an explosion may take the form of a detonation (i.e., an expanding flame that proceeds at a speed in excess of the speed of sound in air) or a deflagration (i.e., an expanding flame that proceeds below the speed of sound in air). In a detonation or deflagration, the destructive forces travel at high speeds, rendering typical fire mitigation technologies ineffective. In a detonation or deflagration, a flame may be released from the system in a dynamic manner; therefore, the flame may take any number of shapes. For example, the flame may be released in the form of a generally expanding conical shape as it moves away from the enclosure. The present disclosure may be used with any shape of flame. In general, a flame being released from the system may be referred to as a “flame ball,” and it may be illustrated figuratively as a circular shape. The term “flame ball”; however, is not restricted to any particular (e.g., spherical or round) geometry, regardless of how the flame is illustrated.
Most materials handling, processing, and storage equipment is not designed to resist the pressure of an explosion. To survive a deflagration, for example, processing and storage equipment typically must be designed to resist the maximum pressure (Pmax) developed by the combustion process. Such design may be prohibitively expensive, however, because Pmax may exceed 75 psig (5.2 bar) in typical cases. Therefore, to address combustion, a process or storage enclosure may be provided with a pressure release device, an explosion venting system, flame arrestor system, or flameless venting system, which will allow the pressure and/or a flame of an explosion to escape the enclosure. Alternatively, a process or storage enclosure may be provided with an explosion suppression system designed to prevent an explosion from occurring. These and other explosion protection/prevention measures are described generally in the text below. Known explosion protection/prevention measures include, for example, the commercially available explosion suppression and chemical isolation systems offered by BS&B Safety Systems. Exemplary BS&B systems include the BS&B Explosion Venting IQR System™; the BS&B Spark Detection & Extinguishing (“SDE”) Systems; and various BS&B explosion vents, including the VSB™, VSP™ VSS™, VSE™, EXP™, EXP/V™, EXP/DV™, LCV™, HTV™ vents.
An explosion venting system provides a pressure release device or an explosion vent as part of the process or storage enclosure. The explosion vent may include an explosion panel, such as those described in co-owned U.S. Pub. Nos. 2005/0235584 and 2007/0181183, the contents of each of which are hereby expressly incorporated by reference in their entirety. An explosion vent may also be provided with a rupture disk, such as those described in co-owned U.S. Pat. Nos. 6,792,964, 6,178,983, and 6,446,653, the contents of each of which are hereby incorporated by reference in their entirety. Pressure release devices and explosion vents are described throughout the present disclosure. Principles of the disclosure may be used with any mechanism by which the effects of an explosion may be vented or released from a system.
Combustion within the enclosure may create an increased pressure (i.e., overpressure), which in turn can lead to opening of the pressure release device or explosion vent. When an explosion vent opens, a flame may be released from the enclosure. The flame may be released directly to the atmosphere. Alternatively, if the pressure release device or explosion vent is deployed within a building or structure, a duct may be used to direct the flame away from the enclosure, e.g., to the exterior of the building or structure. An explosion or pressure venting system may do little to mitigate a flame, a pressure wave, or particulates resulting from the combustion.
FIG. 1 illustrates a flame being emitted from an enclosure by way of an explosion venting system. The exemplary enclosure illustrated in FIG. 1 is a cylindrical dust collector; however, the present disclosure comprehends any number of other process or storage enclosures, including enclosures open, at least in part, to the environment. As discussed above, a combustion may lead to the opening of a vent through which a flame may be emitted. FIG. 1 illustrates a point in time after the vent 3 has opened and while a flame 1 is being emitted. As shown in FIG. 1, the flame 1 has a reach R. In one application, a flame may have a reach of up to 20 feet. In another application, a flame may have a reach of up to 100 feet or more. The flame 1 may have a dynamic shape, with an expanding diameter D, which may expand to around half of the reach R. Although the term “diameter” is used, and the flame is depicted in FIG. 1 as being round, the disclosure is not limited to flames having a circular or other round cross-section. As illustrated in FIG. 1, the flame 1 poses a safety hazard to both personnel and equipment within its reach R. The temperature of a typical flame can reach in excess of 1000 degrees Fahrenheit within a fraction of a second—too hot for human survival and too fast for personnel to remove themselves from harm.
A flame arrestor is a passive flame mitigation device, which may be provided as part of the process or storage enclosure. A flame arrestor may be comprise a component such as a coiled-ribbon-type mesh, woven metallic mesh, or ceramic matrix, which is designed to provide a small flow path. When the flame passes through the small flow path, it tends to be suppressed or extinguished. A flame arrestor is typically deployed in a combustible gas or vapor application. A flame arrestor may provide effective mitigation of a flame, thereby acting as a barrier to the flame's progress. As the size of the enclosure is increased, the flame arrestor must also be increased. Thus, for large enclosures, a flame arrestor is typically a heavy device requiring a significant amount of space for installation. Flame arrestors may also require extensive maintenance. The flame arresting components (e.g., mesh) must be maintained in clean condition. Built-up process material on the arresting components may impair performance. For that reason, flame arrestors may not be suitable for use in a dusty environment, which may cause blockage of the flame arresting components—resulting in a reduced flow rate capability and reduced heat absorption capability. In addition, passive flame mitigation devices, like flame arrestors, might not completely extinguish a flame.
A flameless venting system provides a combination of an explosion vent and a flame arrestor, and is designed to absorb the flame arising from combustion. Depending on the design of the flameless venting device, it may mitigate the flame, reduce a pressure pulse emitted by the combustion, and absorb some or all of the particulates arising from, e.g., a combustible dust explosion. A flameless venting system suffers from similar drawbacks as a flame arrestor system: it may be heavy, require a large amount of space for installation, and must be maintained clean from material buildup. In addition, a flameless venting system may require significant refurbishment or even replacement after exposure to a flame (i.e., after activation).
An explosion suppression system does not require the opening of any venting devices in a process or storage enclosure. An explosion suppression system is provided with a device to prevent the full development of an explosion, thereby preventing formation of a flame and associated pressure rise that would otherwise need to be released to the environment. Such a device may include an explosion suppression agent release device, which can release or inject an explosion suppression agent into the enclosure. Explosion suppression systems may be costly. Moreover, an explosion suppression system may rely on numerous suppression agent injection points, which multiply the cost. In addition, an explosion suppression system may not eliminate the potential for a flame to be emitted. Particularly where the process or storage enclosure is open to the environment, a flame may be emitted despite the activation of an explosion suppression system.
In light of the foregoing, there is a need for a flame mitigation system, which reduces the severity of a flame resulting from an explosion, while reducing cost. The flame mitigation system of the present disclosure achieves these, or other, advantages.